Add asyncTransfer in PullPayment

[wanglingsong]

🎉 Description

• 📈 This is a feature request.

📝 Details

Is it possible to add asyncTransfer method to allow payment transfer between payees?

    function asyncTransfer(address _from, address _to, uint256 amount) internal {
        payments[_from] = payments[_from].sub(amount);
        payments[_to] = payments[_to].add(amount);
    }

[frangio]

Hi @wanglingsong! Thank you for the suggestion.

We are working on a fix for #901 to make PullPayment more trustless, and unfortunately I don't think this feature will make sense after this fix.